Update 2:

Here is a first simple solution. Anybody needs a starting point:

#!/usr/bin/env perl

use strict;
use warnings;
use feature qw(say);
use Data::Dump;
use Lingua::Stem qw(stem);

undef $/;

my $text = <DATA>;

say $text;

$text = lc $text;

$text =~ s/\n+/ /g;

say $text;

$text  =~ s/[:;'!?.,]+//g;

say $text;

my @words = split / /, $text;

dd \@words;

Lingua::Stem::set_locale('de');

say Lingua::Stem::get_locale;

my $stems = stem(@words);

dd $stems;

my %vocabulary = map {$_ => 1} @$stems;

dd  \%vocabulary;

say scalar keys %vocabulary;


__DATA__
Ich Bin Der Geist, Der Stets Verneint!
Und Das Mit Recht; denn alles, was entsteht,
Ist wert, daß es zugrunde geht;
Drum besser wär's, daß nichts entstünde.
So ist denn alles, was ihr Sünde,
Zerstörung, kurz, das Böse nennt,
Mein eigentliches Element.
[download]

It isn't so easy as one might think: Simply counting the words with wc doesn't return the vocabulary. And Lingua::Stem thinks that Ist and ist are different stems for example. And how to filter out the real text from sources which contain a preface, index, bla? And so on.

Some may ask why i waste my time with this issue. It has to do with politics. As this isn't a forum about politics i skip the details.

I was a little bit inspired by what Jill Lepore analogously wrote about facts in her splendid book These Truths: A History of the United States about facts: "Show me yours and i'll show you mine." Basically the same game that we played with our cousins when we were nasty little boys. Discussion later.